A global positioning system (GPS) system determines a precise position on the surface of the earth by measuring the signal propagation time from multiple orbiting satellites. Today's newer high performance GPS receivers used for Real Time Kinetic (RTK) and surveying applications are capable of achieving centimeter level 2-D position accuracy using two methods: (1) Locking to the carrier frequency as well as determining the on-time signal point; and (2) Using Differential GPS (DGPS) to cancel atmospheric effects. Further, each satellite has a precision clock in it which is accurately synchronized by ground control stations. Thus, a side benefit of these navigation systems is that the Earth is covered with a precise time synchronization signal.
With traditional time synchronization, nanosecond level synchronization is typically achieved using a low-cost GPS receiver and a timing processor to improve the accuracy of a stable Oven Controlled crystal Oscillator (OCXO). More specifically, the GPS receiver is used in conjunction with the oscillator (OCXO) and the timing processor to control or “discipline” the oscillator (OCXO) to the more accurate frequency reference derived from the satellite signal. The timing processor commands the GPS to operate in favorable modes for precision timing instead of positioning or navigation. The 1 Pulse Per Second (1PPS) signal from the GPS is used as a reference to phase lock the stable local oscillator. A voltage controlled oscillator (VOCXO) is used so it can be adjusted in phase relative to the 1PPS reference.
This traditional time synchronization, however, does not achieve the desired picosecond level performance because the 1PPS output is a composite from many pseudorange measurements from all of the satellites in view and because the on-time point is derived from the demodulated spread spectrum signal. The most precise spread spectrum signal from these satellites, the P code, is only 10 MHz in bandwidth, limiting the accuracy to a few nanoseconds at best.